Current dogma suggests that all hematolymphoid lineages are derived from a common ancestor, the hematopoietic stem cell (HSC). HSCs are believed to be the only cells with long-term self-renewal capacity in the bone marrow (BM), and are generally regarded as the cell of origin for continuous multi-lineage blood production during adult life. Evidence supporting this HSC-centric paradigm has been acquired through decades of work based largely on the use of functional assays involving transplantation. However, it is unclear to what extent functional characteristics of cells assayed under transplantation conditions are shared with cells driving non-transplant native hematopoiesis. Because of a historical lack of tractable systems, the mechanistic nature of non-transplant blood production has remained largely unexplored. To address this limitation, my laboratory has developed a novel experimental system in mice where cells can be uniquely and genetically labeled in situ. Using this system, clonal fate of multiple hematopoietic populations can be tracked over time and across lineages, for the first time, in a native context. Our preliminary findings with this model have revealed surprisingly unique features of unperturbed hematopoiesis. Among other things, our data demonstrate that long-term native hematopoiesis is mainly driven by waves of progenitor recruitment and that HSC contribution during this process is minimal. Thus, we hypothesize that native hematopoiesis is driven by fundamentally different mechanisms as transplantation. In this proposal, we aim to extend our earlier findings and to provide comprehensive insight into the biology of blood production in situ. Specifically, we will test whether HSCs can be recruited into productive hematopoiesis via injury, stress, or infection. We will also test whether progenitor recruitment can exhaust during the aging process. Additionally, we will revisit the existence of classical progenitor populations and lineage relationships using our clonal strategy. Completion of this project would elucidate the basic mechanisms underlying blood production and provide insight into stem cell dynamics during disease processes.